1. Field of the Invention
The present invention relates to an image forming apparatus such as a printer, a facsimile, and a copier.
2. Description of the Related Art
The conventional image forming apparatus shown in FIG. 7A includes a transferring unit 150 that transfers a toner image onto a surface of a recording sheet P; a fixing unit 180 that fixes the toner image to the recording sheet P; and a conveying unit 175 that conveys the recording sheet P from the transferring unit 150 to the fixing unit 180. The recording sheet P is a recording medium such as a paper sheet. The conveying unit 175 includes two rollers 177 and 178 and a conveying belt 176 that is supported by the rollers 177 and 178. The conveying unit 175 conveys the recording sheet P with the recording sheet P placed on an upper surface of the conveying belt 176 (hereinafter, “conveyance surface”) so that the recording sheet P comes into contact with a fixing-unit-entrance guiding plate 183 at a fixed angle. The fixing-unit-entrance guiding plate 183 is arranged in front of a nip in the fixing unit 180 (hereinafter, “fixing nip”). The recording sheet P is then conveyed to the fixing unit 180, guided by the fixing-unit-entrance guiding plate 183. Because the height and angle of the conventional fixing-unit-entrance guiding plate 183 is fixed, it is difficult to maintain high smoothness when conveying various types of irregular recording sheets, such as oversize sheets having margins with crop marks, and postcards, thin sheets, and thick sheets.
If the recording sheet P is a large sheet having a length longer than the distance between the nip in the transferring unit 150 (hereinafter, “transferring nip”) and the fixing nip, when a leading edge of the recording sheet P enters the fixing nip with the recording sheet P being flat, a vibration is likely to occur at the transferring nip and, therefore, a distorted image is likely to be formed. To solve the problem, the conveyance surface of the conveying unit 175 is arranged beneath an imaginary straight line drawn between the transferring nip and the fixing nip so that the impact that occurs when the leading edge enters the fixing nip cannot be transmitted to the transferring nip. In contrast, if the length of the recording sheet P is shorter than the distance between the transferring nip and the fixing nip, the recording sheet P is conveyed only by the force produced by the conveying unit 175. If the conveying unit 175 is arranged in the above-described manner such that the conveyance surface is beneath the imaginary straight line, the recording sheet P that is an extremely thick sheet may exit the transferring nip in a rigid manner and form a “bridge” between a transferring-unit-exit guiding plate 157 near the transferring nip and the fixing-unit-entrance guiding plate 183. In some cases, a paper jam occurs because there is no contact between the recording sheet P and the conveyance surface of the conveying belt 176 or because there is not enough force to convey the recording sheet P up to and along the fixing-unit-entrance guiding plate 183.
If the fixing-unit-entrance guiding plate 183 and the conveyance surface of the conveying belt 176 make a large angle, the recording sheet P is likely to be in contact with the conveying belt 176 and, therefore, the recording sheet P is easy to enter the fixing-unit-entrance guiding plate 183. Therefore, such paper jams are prevented. However, as shown in FIG. 7B, if the recording sheet P is a thin sheet with a curled leading edge, the recording sheet P is conveyed along the fixing-unit-entrance guiding plate 183 with the leading edge curved upward in noncontact with the fixing-unit-entrance guiding plate 183. The recording sheet P then enters the fixing nip with the leading edge being pointing away from the fixing nip. As a result, a crease is made in the recording sheet P.
The following image forming apparatuses are widely-known that can convey a thick small sheet, such as a postcard, at a high smoothness of conveyance while suppressing vibration of the recording sheet at the transferring nip.
FIG. 10 is an enlarged schematic diagram of a conveying unit 500 and relevant parts included in an image forming apparatus disclosed in Japanese Patent Application Laid-open No. 2001-240268. The conveying unit 500 includes a first unit and a second unit. The first unit includes a first conveying belt 501 that endlessly rotates in a counterclockwise direction in the plane of paper of FIG. 10 and a first roller 502 and a second roller 503 that support the first conveying belt 501. The second unit includes a second conveying belt 504 that endlessly rotates in the counterclockwise direction and a third roller 505 and a forth roller 506 that support the second conveying belt 504. The second roller 503 and the third roller 505 are arranged coaxially, surrounding a single shaft. They can run idle. The first unit swings about the shaft with the shaft being the fulcrum. The conveying unit 500 is designed so that both sections can swing about the fulcrum at or close to the center of the unit in the sheet conveying direction. A guiding plate 507 is fixed to the first unit. The guiding plate 507 guides the recording sheet to a nip between a heat roller 508 and a pressure roller 509 that are included in the fixing unit. The guiding plate 507 swings together with the first unit. If the recording sheet P is a regular sheet, the conveying unit 500 receives the recording sheet P in such a manner that the first unit and the second unit are arranged substantially in a straight line as shown in FIG. 10 and conveys the recording sheet P that is guided by the guiding plate 507 to the entrance of the fixing nip.
FIG. 11 is a schematic diagram that depicts an arrangement of the conveying unit 500 when a large thick sheet is used as the recording sheet P. If the conveying unit 500 receives the large thick sheet that has a length longer than the distance between the transferring nip (not shown in FIG. 11) and the fixing nip, a left side of the first unit swings up. The thick sheet is then guided toward a position on the surface of the heat roller 508 that is located in front of the fixing nip. After coming into contact with the heat roller 508, the leading edge of the thick sheet bends and then enters the fixing nip. Because of the bending at the leading edge, the force that is applied to the leading edge using the fixing nip to convey the thick sheet is not significantly transmitted to the transferring nip between which the trailing-edge side of the thick sheet is inserted. As a result, the formation of distorted images at the transferring nip is prevented.
FIG. 12 is a schematic diagram that depicts an arrangement of the conveying unit 500 when a postcard is used as the recording sheet P. When the conveying unit 500 receives the postcard, the left side of the first unit swings down and the trailing edge of the postcard abuts against the surface of the first conveying belt 501. Because the trailing edge of the postcard abuts against the surface of the first conveying belt 501, the smoothness in conveying of postcards is improved.
FIG. 13 is an enlarged schematic diagram of a conveying unit 600 and relevant parts according to an image forming apparatus disclosed in Japanese Patent Application Laid-open No. 2002-2997. The conveying unit 600 is designed so that both sections can swing about a fulcrum at or close to the center of the unit in the same manner as the conveying unit 500 is designed. In contrast to the conveying unit 500, a guiding plate 607 of the conveying unit 600 is arranged near the fixing nip in a fixed manner.
FIG. 14 is a schematic diagram that depicts an arrangement of the conveying unit 600 when a large thick sheet is used as the recording sheet P. If the conveying unit 600 receives the large thick sheet that has a length longer than the distance between the transferring nip and the fixing nip, the center of the conveying unit 600 lowers as shown in FIG. 14 by swinging of the first unit, so that the center of the thick sheet that is placed over the two conveying belts bends at a steep angle. Because of this bending, the force that is applied to the leading edge using the fixing nip to convey the thick sheet is not significantly transmitted to the transferring nip between which the trailing-edge side of the thick sheet is inserted. As a result, the formation of distorted images at the transferring nip is prevented.
FIG. 15 is a schematic diagram that depicts an arrangement of the conveying unit 600 when a postcard is used as the recording sheet P. If the conveying unit 600 receives the postcard, the left side of the first unit swings down and the trailing edge of the postcard abuts against the surface of the first conveying belt 501 in the same manner as in the conveying unit 500. Because the trailing edge of the postcard abuts against the surface of the first conveying belt 501, the smoothness in conveying of postcards is improved.
In the image forming apparatuses disclosed in Japanese Patent Application Laid-open No. 2001-240268 and Japanese Patent Application Laid-open No. 2002-2997, each of the conveying units 500 and 600 is designed so that both sections can swing about the fulcrum at or close to the center of the unit. This design, in turn, increases the number of parts and manufacturing costs.
Furthermore, the image forming apparatus disclosed in Japanese Patent Application Laid-open No. 2001-240268 may form a snake-like line when a large thick sheet is used as the recording sheet P. When the thick sheet comes into contact with the heat roller 508 at the position that is located in front of the fixing nip (hereinafter, “contact position”), as shown in FIG. 11, toner unfixed to the thick sheet may be rubbed against the surface of the heat roller 508 along the path from the contact position to the fixing nip. This creates a snake-like line running in the sheet conveying direction.
In the image forming apparatus disclosed in Japanese Patent Application Laid-open No. 2002-2997, a paper jam may occur when a postcard is used as the recording sheet P. To obtain a force large enough to convey the postcard from the first conveying belt as definitely as possible, the first conveying belt 501 is preferably sloped as steep as possible with the left side down. However, as the slope of the first conveying belt 501 gets steeper, the angle between the postcard and surface of the guiding plate 507 decreases. If the angle is too small, the leading edge of the postcard fails to turn the corner and the trailing edge of the postcard slips against the first conveying belt 501. As a result, a paper jam occurs.
In the image forming apparatus disclosed in Japanese Patent Application Laid-open No. 2002-2997, if a thin sheet, such as a regular sheet or a thin sheet, that has a the slightly curled leading edge, as shown in FIG. 7B, is used as the recording sheet P, there is possibility that the recording sheet will be folded inwardly or a crease is made at the leading edge. As shown in FIG. 7B, when the regular sheet or the thin sheet that has a slightly curled leading edge is conveyed to the fixing unit along the gentle slope, the fixing nip is likely to receive the recording sheet with the backside of the leading edge upward; therefore, a fold or a crease is likely to be made. In contrast, when the regular sheet or the thin sheet that has a slightly curled leading edge is conveyed to the fixing unit along a steep slope, the curled leading edge comes into contact with the contact position of the roller that is located in front of the entrance of the fixing nip and then turns its moving direction toward the fixing nip. The recording sheet moves from the contact position to the fixing nip with the curled leading edge abutting against the surface of the roller. This means that, if regular sheets and thin sheets, which are likely to have slightly curled leading edges, are used as the recording sheet P, it is preferable to set the fixing-unit-entrance guiding plate 183 to a steep slope. However, in the image forming apparatus disclosed in Japanese Patent Application Laid-open No. 2002-2997 shown in FIGS. 13 to 15, the angle of the guiding plate 507 is fixed even when a recording sheet having a different thickness is conveyed. If the fixed angle is not appropriate for the recording sheet, there is possibility of folding and creasing at the recording sheet's leading edge.